1. Field of the Invention
The invention relates to a pressure transducer for a pressure sensor for determining at least one pressure in a process media having a housing, a separating diaphragm, at least one first pressure-sensing element and at least a first seal, wherein the separating diaphragm separates the process media from the contact media, the contact media conveys the pressure of the process media determined by the separating diaphragm to the first pressure-sensing element, the first seal houses the first connection element and wherein the housing, the separating diaphragm and the first seal form a first pressure chamber. The invention relates further to a method for monitoring the state of such a pressure transducer as well as a pressure sensor that is provided with such a pressure transducer.
Pressure transducers of the type being discussed here have been known for a long time and are used to transform the pressure to be determined in a process media into a measuring signal that can be further processed, for example, is displayed or is used in the broadest sense in the scope of process control tasks.
2. Description of the Prior Art
The physical effects that are used basically for measuring a pressure in a process media or, respectively, in the first pressure chamber of the pressure transducer and to convert the pressure into a corresponding parameter are very different and are based either on mechanical or hydrostatic principles, but often on methods that show an electric effect that can be evaluated (e.g. elastic pressure-sensing probes, electrical measurement of distortion, capacitive measurement of deflection, deformation measurement using a strain gauge, inductive or capacitive distance measurement or piezo-electric, magnetoelastic or resistor pressure-sensing probes). The exact method for pressure conversion is not important in the scope of the invention described here. It is also of no significance, whether the pressure transducer is a transducer for absolute, reference or relative pressure measurement; all considerations shown here can be applied for all of the mentioned types of pressure transducers.
The embodiment described above of a pressure transducer of the sort being discussed here with a separating diaphragm that separates the process media found outside of the pressure transducer from the interior of the pressure transducer exists primarily for protecting the interior of the pressure transducer, in particular, however, the pressure-sensing element of the pressure transducer, from aggressive process media. For this reason, the separating diaphragm is generally made of a very non-corrosive material such as stainless steel, Hastelloy or Tantalum, wherein the separating diaphragm has to be very thin, for example in a range of 30 μm to 50 μm, so that conveying the pressure from the process media to the—mostly liquid—contact media in the first pressure chamber can occur without being influenced by the separating diaphragm, so that the dynamics of the pressure transducer are reduced as little as possible.
Depending on the range of application, pressure transducers can be exposed to high mechanical loads, for example through pressure impulses, temperature fluctuations and chemically aggressive process medium, so that the very thin, and thus mechanically sensitive separating diaphragm, can be damaged to the point that is becomes leaky and the process media can no longer be kept away from the interior of the pressure transducer. The intrusion of a chemically aggressive process media into the interior of the pressure transducer usually results in the destruction of the pressure transducer, which often leads to significant disturbances within the entire process, since the state variables supplied by the pressure transducer or, respectively, the pressure sensor in which the pressure transducer is applied, generally retroact on the process in the scope of controlling or regulation.
The consequences of a process media penetrating into the pressure transducer could, however, be much more serious, in particular when the process media is inflammable or explosive. In this case, there is a risk of the process media igniting on the first pressure-sensing element in the first pressure chamber, if it is connected to electrical potentials, which is the case for most pressure transducers. The greatest risk is that the flames from the process medium ignited on the first pressure-sensing element in the first pressure chamber extend beyond the first pressure chamber into the outer process pipelines, which can lead to an explosion of all areas of the supply and conveying process with process media. Such a spread of flames can be prevented with appropriate constructive measures in the area of the first pressure chamber; pressure transducers having such a “flame barrier” are known as “ex-d” explosion-proof.
However, further defects are known from experience that are not protected by the above-mentioned safety mechanism. A documented, dangerous defect exists in that the first seal of the first pressure chamber becomes leaky, so that the contact media can leak out of the first pressure chamber and, in the case of additional damage to the separating diaphragm, an inflammable process media can penetrate through the first seal, unobstructed by the first pressure chamber into the area of the pressure transducer on the other side of the first seal, opposite the first pressure chamber. On this side of the first seal, opposite the first pressure chamber, there is normally an electric circuit—belonging to the pressure sensor—, so that there is a greater chance here of the process media being ignited. The inflammable process media has enough space here to be ignited in such amounts that the safety mechanism described above no longer guarantees protection against a breakthrough of the explosion in the area of the first pressure chamber. In particular, feedthroughs in the first seal in which connection elements are fed that allow for e.g. an electrical connection of the first pressure-sensing element from outside of the first pressure chamber, are weak points that lead to leaking.